Old Style and New Style dates

Treaty of Lübeck (1629), with the Gregorian day (22) directly above the Julian day (12), both before the name of the month, May. The treaty was made between Catholic parties who had already begun using the Gregorian calendar, and Protestant parties who had not.

Old Style (or O.S.) and New Style (or N.S.) are terms used in English language historical studies for two reasons. The first reason is that the method of dating that is most widely used around the world today, the Gregorian calendar, has not always been used. The second is that 1 January has not always been the first day of the year. Both of these conventions changed not very long ago. So when a date is given in a history book (or an old book), we need to know whether it is in the modern New Style or the traditional Old Style. During the time of the changeover, people would give both dates. Even today, when historians are writing about an event in those times, they often give the date as it was used at the time but also give the modern equivalent for your convenience.

The reason for changing the calendar was that experts in the early 1700s realised that that there is a mistake in the Julian calendar, that it adds too many leap years. This meant that the date of Easter was being calculated wrongly. So they designed a new calendar that corrected this error. Pope Gregory XIII declared that this new calendar should be used from 1753 onwards. But only Roman Catholic countries accepted this ruling: Protestant and Eastern Orthodox countries refused to have the Pope tell them what to do. So even in Europe, the change happened at different times. For example, it was not until 1752 that Great Britain and its colonies changed over to the new calendar, changing the start of the year to 1 January at the same time.[1][2][3] Russia changed in 1918, after the 'October' Revolution.[4]

The Latin words for O.S. are stili veteris or stilo vetere. These terms are used in some books worldwide. They can be shortened to st.v.[5]

↑Stockton, J.R. Date Miscellany I: The Old and New Styles "The terms 'Old Style' and 'New Style' are now commonly used for both the 'Start of Year' and 'Leap Year' [(Gregorian calendar)] changes (England & Wales: both in 1752; Scotland: 1600, 1752). I believe that, properly and historically, the 'Styles' really refer only to the 'Start of Year' change (from March 25th to January 1); and that the 'Leap Year' change should be described as the change from Julian to Gregorian."

↑Spathaky, Mike Old Style New Style dates and the change to the Gregorian calendar. "increasingly parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the New Style 1734 started even though the Old Style 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Old or the New Style reckoning. The date should either be written 20th January 1745 OS (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."